The check engine light is on in my 1989 Ford E250 Econoline van. I heard from a mechanic (who changed my oil) that there's a "key cycling" process that will cause the light to flash in a certain pattern that encodes the cause of the check engine light.

How does "key cycling" work and where can I find a decoding scheme so I can tell what the problem is?

EDIT: Based on Jaime's answer, I consider the second half of this question to be answered. However, I still haven't gotten the key cycling to work (even after trying Dave's suggestions).